Many nitride-based semiconductor devices use a sapphire substrate. However, a sapphire substrate is expensive, is too hard to manufacture chips, and has low electric conductivity. Furthermore, a sapphire substrate may not be easily manufactured at a large size due to its warpage at high temperatures, such as during epitaxial growth, and due to its low thermal conductivity. In order to prevent the above problems, nitride-based semiconductor devices using a silicon (Si) substrate instead of a sapphire substrate have been developed. Since a Si substrate has higher thermal conductivity than a sapphire substrate, the Si substrate is not warped greatly at a high temperature for growing a nitride thin film, thereby making it possible to grow a large thin film on the Si substrate.
However, when a nitride thin film is grown on a Si substrate, a dislocation density may be increased due to a mismatch in lattice constants between the Si substrate and the nitride thin film and cracks may occur due to tensile stress which is generated by mismatch in thermal expansion coefficients between the Si substrate and the nitride thin film. Accordingly, many types of buffer structures are studied to grow a nitride thin film, which has high crystallinity and has no crack, on the Si substrate.